I'm DMing a 4e dnd game. The party is level 2 and contains a Dragonborn Warlord, Shifter Ranger (Two-Blade) and a Dwarf Summoner Wizard. As you can see, no defender. I'm aware that the Warlord is defenderish and I think that the Wizard may be able to sufficiently fill this role that the addition of an NPC is unnecessary. Thoughts?
No, a summoner wizard cannot replace a defender.
Summons won't even come close to replacing a defender. First, summon powers are all dailies, so at level 2 unless the party is only having 1 fight per day then the wizard is not going to have a summon for every fight. Second, summons lack any method of actually defending. They have no marks, and they usually can't make opportunity attacks, so they have no way of getting monsters to actually attack them.
That said, while the dwarf wizard is probably going to be mostly dead weight, the warlord & ranger should do quite well for themselves. You shouldn't toss in any NPCs to help defend for a level or two; just use the standard XP budgets for a 3-person party and see how they do. You should only toss in a defender NPC if they're taking too much damage and are having trouble making it through 3-4 encounters per day without running out of surges.